The present invention relates to an integrated circuit device comprising an inductor with high quality coefficient. The quality coefficient or quality factor of an inductor, generally dubbed Q, is defined as the ratio between the imaginary part and the real part of the impedance of the inductor. This coefficient can be measured with the aid of a network analyser.
The regular reduction in the dimensions of the active devices such as a transistor in integrated circuits, in particular on silicon, is necessary to allow an increase in the density of integration and leads to an improvement in their frequency performance. Nowadays, radio frequency functions such as a low noise amplifier (LNA), a mixer, a power amplifier or else a voltage controlled oscillator (VCO) can be integrated for frequencies above 2 GHz.
A silicon-based integrated circuit comprises a substrate of intrinsic silicon (pure silicon) into which have been introduced impurities such as atoms from group III of the Periodic Table (boron B, gallium Ga, etc.), and P doped silicon is then obtained.
A layer of insulating material, SiO2 for example, is interposed between the substrate and the inductor. The inductor is made in the form of a spiral situated at the last level of metal which is generally aluminium.
As may be seen in FIG. 1 which is a sectional view of the inductor, this inductor is made in the form of turns Sp deposited above an insulating layer of silicon oxide SiO2 which is itself deposited on a substrate of silicon Si. To take account of the stray effects, a stray coupling is manifested by capacitances Cp representing the capacitance of the insulating layer between the inductor and the substrate and leakage resistances Rp modelling the leakages of energy in the substrate. FIG. 2 illustrates an electrical modelling of the inductor and of its stray components, in which the inductor is modelled by a pure self-inductance L and a series resistance Rs, and the stray capacitances Cp are linked to the leakage resistances Rp.
Thus, if one studies the changes in the quality coefficient Q as a function of frequency f, one observes different behaviour depending on the frequency domain. Curve 2 of FIG. 3 represents these changes in the quality coefficient Q for a real inductance. At low frequency the quality factor Q increases with frequency as in the case of an ideal inductance (Q=L2xcfx80f/Rs) represented by curve 1 in FIG. 3, when it reaches a maximum Qmax for a resonant frequency F0. At high frequency Q then decreases, departing from curve 1 representing an ideal inductance and in doing so increasing the stray capacitances Cp1 and Cp2. This phenomenon manifests the fact that at high frequency these capacitances Cp1 and Cp2 constitute low impedances and that the dissipation of energy in the leakage resistances Rp1 and Rp2 becomes large. The stray elements, namely Cp1, Cp2, Rp1 and Rp2, therefore limit the maximum value Qmax of the quality coefficient and the resonant frequency F0. Beyond F0, the inductor no longer behaves as a true inductor. It is difficult to make an integrated inductor having a high quality coefficient Q since the stray elements are inherent in its construction. The series resistance Rs also contributes to the limiting of the quality coefficient Q. The assembly consisting of the series resistance and stray elements means that, in FIG. 3, curve 2 of the real inductance departs from curve 1 of the ideal inductance having a quality coefficient which increases with frequency both at low frequencies and at high frequencies.
The use of inductances with high quality coefficients is necessary in numerous situations so as to allow the integration of radio-frequency functions (a few GHz) with good characteristics. This is the case more particularly for circuits carrying out the following functions: VCO (voltage controlled oscillator), PA (power amplifier), filters, LNA (low noise amplifier).
In the case of VCOs the reduction in the phase noise is largely dependent on the quality coefficient Q of the LC circuit (made up of inductances and capacitances) of the oscillator. The circuits of the PA type, the gain and the efficiency are greatly improved if the passive elements, such as the inductor, exhibit minimal losses. Furthermore, the integration of passive filters is made easier by inductors of high quality coefficient.
Systems are known which make it possible to increase the quality coefficient at higher and higher frequencies by acting on various parameters:
increasing the thickness of the metal layer;
choosing a less resistive metal, for example employing copper instead of aluminium;
moving the metal layer further from the substrate;
locally modifying the resistivity of the substrate by using a patterned substrate.
These modifications relate to the technology of integrated circuits.
In the prior art, a system is known for improving the quality coefficient by eliminating the series resistance Rs. To do this, a new active circuit is introduced which produces a negative resistance in series with the self-inductance L and the series resistance Rs. However, the proper operation of this device requires rigorous knowledge of the values of the components, especially an absolute value of the negative resistance not exceeding the value of the series resistance Rs on risk of engendering an oscillation phenomenon. The stray elements limiting the value of the quality coefficient Q and the resonant frequency F0 cannot be sufficiently eliminated using this system.
An embodiment aims to afford a solution to this problem by reducing the influence of the stray capacitances and of the leakage resistances in relation to the inductance.
An embodiment is to improve the quality coefficient Q.
An embodiment is an integrated circuit comprising an inductor made at a metallization level of the circuit and a buried layer situated in the substrate of the integrated circuit under the said inductor.
In an embodiment, the integrated circuit includes a connector linking the inductor to the buried layer and configured in such a way as to ensure the same potential in terms of dynamic response between the inductor and the buried layer. The signal at the output of the inductor is reproduced identically, same phase and same amplitude, at the level of the buried layer. The terminals of the stray capacitances, more particularly of the stray capacitance Cp2, are thus at the same potential in terms of dynamic response, thus making it possible to render the capacitance Cp2 invisible by the circuit. The injurious effects of this capacitance therefore no longer effect the inductor. It is thus possible to increase the frequency of operation of the integrated inductor.
Preferably, the connector includes a transistor in a follower type arrangement made in the substrate. Advantageously, this transistor in a follower type arrangement is a bipolar transistor biased in emitter follower mode. The xe2x80x9cfollowerxe2x80x9d characteristic makes it possible to xe2x80x9ccopyxe2x80x9d the signal present on a terminal of the inductor at the level of the buried layer. Furthermore, the leakage resistances Rp are thus now supplied by a current coming from the transistor rather than via the inductor. To avoid extra integration cost and energy consumption due to the active component, namely the transistor, one advantageously uses a transistor which is already implanted in the integrated circuit such as a transistor used as a buffer in a low noise amplifier (LNA) circuit for example.
According to a variant of the invention, the transistor in a follower type arrangement can be a MOS transistor (Metal Oxide Semiconductor) as shown in FIG. 9. This MOS transistor may or may not be associated with a bipolar transistor in a follower type arrangement.
In an embodiment, the connector includes, between the transistor and the buried layer, a resistance of sufficiently low value as to ensure substantially the same potential between the inductor and the buried layer. This low-value resistance makes it possible to avoid any oscillation problems which may come about when the series resistance Rs is very low on the order of 1.5 Ohms or less than this value. The low-value resistance which is added has a value of the order of 5 Ohms. This value is low relative to the value of the leakage resistances Rp which is several hundred Ohms such as 250 Ohms for example. This difference in magnitude guarantees that the transistor is always regarded as a voltage generator imposing a potential on the buried layer substantially identical to the potential of the inductor owing to the fact that a resistance of low value with respect to the load resistance, namely the leakage resistances, is placed between the buried layer and the emitter of the transistor. Thus, this added resistance avoids any oscillations and by reason of its low value keeps the inductor and the buried layer equipotential.
According to an embodiment , the integrated circuit operating at a given working frequency, the transition frequency of the transistor is much greater than the working frequency. Thus, the characteristics of the transistor are not degraded, in particular its transconductance gain, for a given working frequency.
Moreover, the buried layer is preferably a layer of patterned type. It may also be non-patterned or else made of a metal material. In all cases, this structure minimizes the coupling of the inductor with the substrate.
In an embodiment, the transistor is biased in such a way that the current passing through it is capable of charging stray capacitances appearing between the buried layer and the substrate of the integrated circuit. The transistor bias current is thus fixed in such a way as to obtain a compromise between optimal frequency behaviour of the transistor and reason consumption.
One embodiment is directed to a process for improving the quality coefficient of an inductor in an integrated circuit. Generally the said inductor is linked to a buried layer in the substrate of the integrated circuit in such a way as to render the inductor and the buried layer equipotential in terms of dynamic response.
A transistor can be connected in a follower type arrangement between the inductor and the buried layer so as to maintain the same potential between the inductor and the buried layer.
The transistor and the buried layer can be linked with the aid of a resistance.
Advantageously, the transistor is biased in such a way that the current passing through it is capable of charging stray capacitances appearing between the buried layer and the substrate of the integrated circuit.